Cryogenic storage and transfer is widely used in commercial, medical and aerospace applications. Cryogenic systems require storage reservoirs or tanks, and also require some type of delivery system in the form of feedlines, pipelines or piping system to deliver the cryogen to the functional use. Cryogenic fluids are extremely cold and have low boiling point temperatures; therefore it is difficult to store them without significant fluid loss due to boiloff. One way to reduce the cryogen boiloff is to provide better performing thermal insulation. With ever increasing uses and costs for cryogenic fluids, is the necessity for higher performing thermal insulation systems. Thermal heat leaks are attributed to three modes; conductive, radiative and convective. One method of reducing thermal losses is to operate in an evacuated space, thus eliminating all convective losses, which offers significant reduction in system thermal losses. Reducing additional system losses requires reduction in both radiative and conductive losses.
NASA and commercial launch vehicles typically use cryogenic propellants such as liquid Hydrogen (LH2) at 20K (−423 F), and liquid Oxygen (LOX) at 90K (−298 F), which require thermal insulation for storage and preservation. Heat leak into cryogenic systems limits launch vehicle payloads, and limits spacecraft capabilities and mission durations. Improvements in cryogen storage and transfer are critical to future NASA and commercial space vehicles and extended duration missions. Insulation on cryogenic feed lines is especially problematic, with traditional insulation on piping offering poor thermal performance. MLI wrapped on cryogenic feedlines performs substantially worse than MLI on larger tanks. As much as 80% of total system heat into a spacecraft cryogenic RCS propulsion system is due to transfer line heat leak. Ground support equipment at the launch pad that transfers cryogenic propellants is also problematic, and large amounts of liquid Hydrogen are lost during ground hold and prelaunch activities. As much as 50% of liquid Hydrogen used for each shuttle launch or about 150,000 gallons was lost during transfer chill down and ground hold. Future cryogenic launch vehicles and GSE equipment can benefit from higher performing piping insulation.
A number of aerospace applications also use cryo-coolers for active system cooling as an aid to further reduce propellant boiloff, ideally achieving zero boiloff for extended missions. These cryo-coolers generally circulate liquid Helium (LHe) at 4K (−452 F), or liquid Neon (LNe) at 27K (−411 F) to intercept environmental heat and circulate it away from the tank. These systems require very good insulation on the plumbing to maintain an acceptable efficiency.
Many industrial and medical applications use hot or cold transfer lines. Food and beverage processing facilities use liquid nitrogen (LN2) at 77K (−321 F) or liquid carbon dioxide (LCO2) at 217K (−69 F) for package pressurizing, inerting and flash freezing for increased product shelf life. Medical facilities and pharmaceutical plants use LN2 or LHe in systems and procedures such as MRI, cryobiology, cryosurgery and blood banking, and for manufacture processing requiring long runs from storage reservoir to multiple use points. Liquid natural gas (LNG) is generally used as a means for transporting natural gas to markets where it is regasified and distributed to consumers as pipeline natural gas. The gas is condensed down to liquid form at low pressure by cooling to approximately 111K (−260 F). This liquid form takes up 1/600th of the volume of natural gas in the gaseous state which makes it attractive from a transport and delivery aspect. Long pipe runs sometimes of many miles are required to deliver the LNG to storage and transport equipment and poor insulation can have a dramatic effect on process efficiency. In many cases, LN2 is used for flow control and purge of LNG systems and therefore improved insulation would be very beneficial. Industrial uses of cryogens are many including Nuclear Magnetic Resonance Spectroscopy (NMR), forward looking Infrared (FLIR) imaging, and superconducting power distribution. High temperature industrial process pipelines also require thermal insulation for efficient operation. Steam and geothermal power plants are industrial applications in which the operating fluid is at high temperatures. Long pipe runs up to many miles are used and thermal management is a critical aspect to process efficiency.
Aerospace, medical and industrial applications can benefit from higher performing insulations especially where there is a large temperature differential from the system to the ambient environment.
Generally in all thermal transfer applications, system thermal performance is an important factor as it equates to cost, mass, thickness, reliability, predictability, durability, efficiency and capabilities. The general types of insulations used in these systems are foams, fiberous blankets, bulk fill, silicas, or multilayer insulation.
Foam and fiberous materials offer reasonable insulation characteristics, though are of relatively high thermal conductivity on the order of 7-20 mW/m-K in high vacuum. When the differential temperature is large, they must be undesirably thick to keep losses to a minimum. At a density of approximately 38 kg/m3 (e.g., foam) the mass becomes high. A bulk fill material such as Perlite (siliceous rock) offers another solution and has a thermal conductivity on the order of 40-100 mW/m-K, though they are very dense at 50-240 kg/m3. Silica materials are also used as insulation materials though have limited application mainly for higher temperature capabilities. Silica has a thermal conductivity of approximately 1.4 W/m-K, and the density is high at 240 kg/m3. Silicas are capable of withstanding temperatures of 1700K (2600 F). Silica aerogels have low thermal conductivities, on the order of 1 to 19 mW/m-K, but are also quite dense at 50 kg/m3.
Multi-layer insulation (MLI) is generally the preferred insulation for in-space or in vacuum application due to the increased performance in vacuum over alternative insulation materials. In a high vacuum environment such as space (<10-2 Pa), or vacuum jacketed pipe, MLI generally offers performance 10 times better than competitive insulation such as foam or silica type blankets or bulk fill. Traditional MLI is a series of alternating layers of low emissivity radiant barriers of metallic or metalized polymer film and thin polyester or silk netting. The netting is used to separate the radiant barriers and prevent interlayer contact. Netting based MLI has a thermal conductivity of approximately 0.25 mW/m-K and a density of 23 kg/m3. This is approximately 28 times better performance than foam insulation. However, traditional MLI insulation for cryogenic propellant feed lines is problematic, and performs typically about ten times worse per area than MLI insulation on a tank. This is representative of capabilities of current state of the art insulation technology for cryogenic piping.
The poor performance of MLI wrapped on feed lines is due to compression of the MLI layers when wrapped on a curved surface, with increased interlayer contact and heat conduction. The compression is dependent on wrap tension and compression due to loading from additional layers. Conventional MLI on large cryotanks has a typical “degradation factor” (from the heat leak predicted by the semi-empirical “Lockheed equation” developed for multilayer insulation) of 1.6; the measured degradation factors for MLI spiral wrapped on pipes can be as high as 20. The MLI performance is not only much worse than expected, but also is difficult to predict due to the uncertainty of increased interface conductance driven by wrap tension and installation workmanship.
Embodiments of the present invention solve a number of the shortcomings of the prior state of the art methods for insulating hot and cold feed lines used in aerospace and commercial applications. Significant reductions in heat leak and fluid boiloff can be realized. Embodiments of the present invention also allow for much greater confidence in predictability and repeatability, and are less dependent on effects of compression due to gravity or installation workmanship. Measured thermal characteristics of the present invention demonstrated a thermal conductivity as low as 0.06 mW/m-K with a density of 32 kg/m3. This is over four times better performance than current state of the art MLI, 16 times better than bulk fill, and 116 times better than foam insulations.